To save and instance to use later and not pay for the CPU/RAM costs you can "Shelve" the instance. (Note you will still have to pay for disk usage when an instance is shelved).
You can shelve an instance from the main instance tab, in the pull down on the right:
You can "unshelve" an instance when you are ready to use it again. It will keep all the same ip addresses (and floating ips):
